1. Field of the Invention
The present invention relates to a display device, a method for manufacturing the display device, and an electronic appliance including the display device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic appliance are all semiconductor devices.
2. Description of the Related Art
A transistor formed over a glass substrate or the like includes amorphous silicon, polycrystalline silicon, or the like, as typically seen in a liquid crystal display device or an EL display device. Although a transistor using amorphous silicon has low field effect mobility, it can be formed over a larger glass substrate. On the other hand, although a transistor using polycrystalline silicon has high field effect mobility, it is not suitable for being formed over a larger glass substrate.
In contrast to a transistor using silicon, attention has been focused on a technique of manufacturing a transistor using an oxide semiconductor and applying such a transistor to an electronic appliance or an optical device. For example, a technique of manufacturing a transistor using zinc oxide or In—Ga—Zn—O-based oxide as an oxide semiconductor, and using such a transistor for a switching element of a pixel in a display device and the like is disclosed in Patent Document 1 and Patent Document 2.
Further, display devices with large display areas have increasingly become popular. Televisions with display screens having a diagonal of 40 inches to 50 inches are becoming common as home televisions and are expected to become more popular in the future. As described above, the field effect mobility of the transistor using an oxide semiconductor is ten or more times as high as that of the transistor using amorphous silicon; therefore, the performance of the transistor using an oxide semiconductor is high enough to allow the use of the transistor as a switching element of a pixel in a display device having a large display area.
Furthermore, when the display devices with large display areas are manufactured, there is a significant problem of signal delay due to resistance of a signal line. For this reason, a material with a low electric resistance value is used for a material of the signal line. In addition, a structure in which an auxiliary electrode is provided in a pixel portion is disclosed in Patent Document 3.